View Full Version : Alternate AR Calibers

03-17-2015, 6:59 PM
I currently own 4 AR's in 5.56. I would like to change 3 of them to accept other calibers, such as 300 blackout and 6.8.

What calibers can I change them to if all I would like to do is change the barrel? I would prefer not to have to change the upper.

03-17-2015, 7:01 PM

03-17-2015, 7:26 PM
Poached from another forum

AR-15, without bolt modification
.17 Remington
.20 Tactical
.20 Practical
.20 Vartag
.204 Ruger
.221 Fireball
.222 Remington
.222 Remington Magnum
.223 Remington (5.56x45mm)
.223 Remington Ackley Improved
6mm TCU
6mm Whisper
6.5mm Whisper
7mm Whisper
7mm TCU
.300 Whisper (.300/221, .300 Fireball)
.338 Whisper

AR-15, with bolt modification
223 WSSM
5.45x39mm (.21 Genghis)
243 WSSM
6mm PPC
6mm WOA
6mm BR Remington
6mm Hagar
6.5mm PPC
6.5 WSSM
6.5 WOA
6.5mm Grendel
6.8x43mm SPC
.30 Herrett Rimless Tactical (6.8x43mm case trimmed to 41mm and necked up to .308; the 6.8mm version of the .300 Whisper)
.30 RAR
300 OSSM
.357 Auto
.35 Gremlin (necked up 6.5 Grendel to 358)
.358 WSSM (various names, but all are some form of a WSSM necked up to 35 caliber, some are shortened to make them big game legal in Indiana)
.458 SOCOM
.50 Action Express
.50 Beowulf

AR-15 using a simple blowback operation
.17 HMR
.22 LR
.22 WMR
30 Carbine
357 Sig
400 Cor-Bon
41 Action Express
10mm Auto
45 GAP
45 Super
45 Win Mag

This list is in no way complete

03-17-2015, 7:55 PM
I would be more concerned about magazine availability first
then ammo

03-17-2015, 8:03 PM
.300 blackout and whisper for only a different barrel. I think .17 wmr and .204 ruger might also get away with it. 6.5 Grendel, 6.8 spc and 7.62x39 also need different magazines and bolt. Here's a link to 637 yard shot on a groundhog with a 6.8

Guardian Mode
03-17-2015, 8:45 PM
I noticed a set of rcbs 6x45 dies on Amazon for like $90 and midway has a 6x45 barrel for $210. Just saying.

Something I've been thinking about trying myself.

Jimi Jah
03-18-2015, 7:36 AM
7.62x39 needs the special BCG, AIM has them for $99. Or, just the bolt for $68.

.223 mags will work if you trim down the mag face under where the bullets face. Otherwise they will hang up.

I was going to build a 300BO, but the ammo is underpowered next to 7.62x39 and costs double or more.

03-18-2015, 11:04 AM
300 Blackout. Requires just a barrel change. You can buy factory loaded ammo but its hard to find in stores. At least for me it is. I reload so its a non issue. Shines in a short barrel ie. AR pistol

6x45. Requires a barrel change. Ive never seen factory loaded ammo but I hear it exist. Low recoil and shoots very straight. Mine is in a 18" BHW barrel and is my primary coyote hunting AR. Damn accurate.

277 Wolverine. Requires a barrel change. I just recently heard about this caliber and do not know much about it. Basically a 6.8 bullet in 223 brass. I have a spare 80% I haven't milled yet and it might go this direction.

25x45. Requires barrel change. Pretty much a 25 caliber in 223 brass. I know nothing of this one either. I considered it but went 6x45 instead.

There are others Im sure as wildcats are endless. Im sure there is a 17-223 and a 6.5mm-223 out there somewhere. Unless you handload, your limited to 300BO and maybe 6x45.

Other calibers require barrel, bolt, mag, etc changes. 50 Beowulf, 458 Socom and (I think) 450 Bushmaster require ejection port modifications to the upper.